1. Field of the Invention
The present invention relates to an LED, and more particularly to an LED driver chip for a car reading light and a state control method thereof.
2. Description of the Prior Art
LEDs are more and more widely used in automotive lighting systems because of the advantages of energy-saving, high efficiency, etc. However, for low-priced and middle-priced cars, the front and rear reading lights in the car generally use an incandescent light bulb with tungsten filaments as a light source.
FIG. 1 is a circuit diagram of a conventional car reading light with an incandescent bulb. A 12 volts power supply, an incandescent light equivalent to a resistor, and a tri-state switch including three connecting ways are provided. When the tri-state switch is pushed to the left, the incandescent light is lit. The current of the incandescent light is determined by the battery voltage divided by the equivalent resistance of the incandescent bulb; when the tri-state switch is pushed to the middle, the current of the incandescent light is controlled by the BCM output. When the BCM output is a low level close to 0 volt, the incandescent light is lit. The current of the incandescent light is determined by the battery voltage divided by the equivalent resistance of the incandescent bulb. When the BCM output is a high level approximate 12 volts, the incandescent light has no current. The light is off. The current of the incandescent light is approximately zero. When the tri-state switch is pushed to the right, one end of the incandescent light is connected to the battery, and the other end is open. The current is 0 ampere, and the incandescent light is off.
The brightness of the incandescent light is determined by the current value passing the incandescent light. The current value is determined by the voltage difference between two ends of the incandescent light divided by the equivalent resistance of the incandescent light. The resistivity of the tungsten filaments of the incandescent light will increase with an increase of temperature. As a result, the brightness of the incandescent light will vary with a temperature change. The jitter of the battery voltage will cause a jitter of the voltage difference between the two ends of the incandescent light, resulting in a flicker of the brightness of the incandescent light. Accordingly, the inventor of the present invention has devoted himself based on his many years of practical experiences to solve this problem.